How would i know if i had breast cancer?

It's subtle: Breast cancer usually announces itself as a dominant lump in the breast and a developing breast cancer usually becomes palpable or visible on imaging before it becomes a major danger to your life. A "rash" on the nipple is another warning.

See a doctor: Need to see a doctor for annual breast physical examination to look for breast lumps. Usually get mammogram yearly starting age 40, and sooner if your doctor's examination feels an abnormality.